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Oklahoma
Geological
Survey
Dr.
G.
Randy
Keller
(405)
325-­‐7968
Director
and
State
Geologist
March
22,
2013
Austin
Holland
(405)
325-­‐8497
Seismologist
The Oklahoma Geological Survey is a state agency for research and public service; charged with investigating the
state's land, water, mineral, and energy resources and disseminating the results of those investigations to promote
the wise use of Oklahoma's natural resources consistent with sound environmental practices.
The
Prague,
Oklahoma,
earthquake
sequence
of
2011,
along
the
Wilzetta
Fault
zone,
included
a
significant
foreshock,
a
main
shock
of
magnitude
5.7
and
numerous
aftershocks.
It
has
been
suggested
that
this
sequence
represents
earthquakes
triggered
by
fluid
injection.
The
Oklahoma
Geological
Survey
(OGS)
has
been
working
with
the
Oklahoma
Corporation
Commission
(OCC)
to
analyze
data
related
to
the
2011
Prague
sequence,
and
recently
the
U.S.
Environmental
Protection
Agency
(EPA)
has
participated
in
reservoir
pressure
analysis
of
the
formations
into
which
water
is
being
injected.
Based
on
analysis
of
earthquake
seismology,
3-­‐D
reflection
seismology,
geologic
mapping,
formation
pressure
data,
and
historical
Oklahoma
earthquake
data,
the
OGS
offers
the
following
observations
and
conclusions:
Observations:
• Relatively
large,
natural
earthquakes
occur
in
Oklahoma,
and
the
U.S.
Geological
Survey
seismic
hazard
map
shows
that
the
seismic
hazard
for
the
Prague/Wilzetta
area
was
finite
(not
zero)
prior
to
the
Prague
sequence.
• Oklahoma
has
experienced
more
than
ten
magnitude
4.0
or
greater
earthquakes
since
the
magnitude
5.0+
El
Reno
earthquake
of
1952.
This
is
statistically
consistent
with
the
Gutenberg-­‐Richter
relationship,
which
describes
the
distribution
of
earthquakes
of
differing
magnitude
over
time.
• Earthquakes
in
the
Prague
area
have
followed
the
normal
Omori
Law
aftershock
time-­‐decay
that
is
typical
of
natural
seismicity.
• The
earthquakes
occurred
on
a
segment
of
the
Wilzetta
Fault
that
is
favorably
oriented
for
earthquakes
to
occur
as
a
result
of
the
predominant
crustal
stress
regime
in
Oklahoma.
• Water
injection
began
in
the
Prague/Wilzetta
area
in
1955,
increased
until
2004-­‐2005
and
has
remained
relatively
constant
since
then.
Some
researchers
have
observed
that
the
earthquake
activity
did
not
increase
over
time
as
injection
increased,
but
rather
occurred
in
a
distinct
“swarm”
more
typical
of
a
natural
event.
• 3-­‐D
seismic
recently
made
available
demonstrates
an
alternative
interpretation
to
the
premise
of
a
fault-­‐bounded
block,
which
is
the
closed
compartment
postulated
by
some
researchers
that
would
result
in
increased
fluid
pressures
due
to
water
injection.
The
3-­‐D
seismic
data
shows
that
the
local
geological
structure
is
not
fault-­‐bounded
on
all
sides,
which
further
suggests
that
water
injection
is
not
being
restricted.
• This
alternative
structural
interpretation
is
supported
by
recent,
direct
pressure
measurements
of
the
geologic
formations
where
water
injection
activities
are
being
conducted.
Measurements,
undertaken
by
the
OCC,
and
reviewed
by
the
EPA,
show
that
the
Hunton
Formation
is
in
fact
under-­‐pressured.
Initial
results
from
a
separate
test
in
the
area
of
interest
indicate
that
the
Arbuckle
Formation
is
also
under-­‐pressured.
Conclusions:
• The
interpretation
that
best
fits
current
data
is
that
the
Prague
Earthquake
Sequence
was
the
result
of
natural
causes.
• The
Prague
Earthquake
Sequence,
as
well
as
other
current
and
historically
active
seismic
areas
in
Oklahoma,
would
benefit
from
further
study,
including
improved
earthquake
monitoring
and
acquisition
of
formation
pressure
data.
• Further
studies
of
seismic
activity
in
Oklahoma
will
result
in
a
better
understanding
of
seismicity
statewide.

Oklahoma
Geological
Survey
Dr.
G.
Randy
Keller
(405)
325-­‐7968
Director
and
State
Geologist
March
22,
2013
Austin
Holland
(405)
325-­‐8497
Seismologist
The Oklahoma Geological Survey is a state agency for research and public service; charged with investigating the
state's land, water, mineral, and energy resources and disseminating the results of those investigations to promote
the wise use of Oklahoma's natural resources consistent with sound environmental practices.
The
Prague,
Oklahoma,
earthquake
sequence
of
2011,
along
the
Wilzetta
Fault
zone,
included
a
significant
foreshock,
a
main
shock
of
magnitude
5.7
and
numerous
aftershocks.
It
has
been
suggested
that
this
sequence
represents
earthquakes
triggered
by
fluid
injection.
The
Oklahoma
Geological
Survey
(OGS)
has
been
working
with
the
Oklahoma
Corporation
Commission
(OCC)
to
analyze
data
related
to
the
2011
Prague
sequence,
and
recently
the
U.S.
Environmental
Protection
Agency
(EPA)
has
participated
in
reservoir
pressure
analysis
of
the
formations
into
which
water
is
being
injected.
Based
on
analysis
of
earthquake
seismology,
3-­‐D
reflection
seismology,
geologic
mapping,
formation
pressure
data,
and
historical
Oklahoma
earthquake
data,
the
OGS
offers
the
following
observations
and
conclusions:
Observations:
• Relatively
large,
natural
earthquakes
occur
in
Oklahoma,
and
the
U.S.
Geological
Survey
seismic
hazard
map
shows
that
the
seismic
hazard
for
the
Prague/Wilzetta
area
was
finite
(not
zero)
prior
to
the
Prague
sequence.
• Oklahoma
has
experienced
more
than
ten
magnitude
4.0
or
greater
earthquakes
since
the
magnitude
5.0+
El
Reno
earthquake
of
1952.
This
is
statistically
consistent
with
the
Gutenberg-­‐Richter
relationship,
which
describes
the
distribution
of
earthquakes
of
differing
magnitude
over
time.
• Earthquakes
in
the
Prague
area
have
followed
the
normal
Omori
Law
aftershock
time-­‐decay
that
is
typical
of
natural
seismicity.
• The
earthquakes
occurred
on
a
segment
of
the
Wilzetta
Fault
that
is
favorably
oriented
for
earthquakes
to
occur
as
a
result
of
the
predominant
crustal
stress
regime
in
Oklahoma.
• Water
injection
began
in
the
Prague/Wilzetta
area
in
1955,
increased
until
2004-­‐2005
and
has
remained
relatively
constant
since
then.
Some
researchers
have
observed
that
the
earthquake
activity
did
not
increase
over
time
as
injection
increased,
but
rather
occurred
in
a
distinct
“swarm”
more
typical
of
a
natural
event.
• 3-­‐D
seismic
recently
made
available
demonstrates
an
alternative
interpretation
to
the
premise
of
a
fault-­‐bounded
block,
which
is
the
closed
compartment
postulated
by
some
researchers
that
would
result
in
increased
fluid
pressures
due
to
water
injection.
The
3-­‐D
seismic
data
shows
that
the
local
geological
structure
is
not
fault-­‐bounded
on
all
sides,
which
further
suggests
that
water
injection
is
not
being
restricted.
• This
alternative
structural
interpretation
is
supported
by
recent,
direct
pressure
measurements
of
the
geologic
formations
where
water
injection
activities
are
being
conducted.
Measurements,
undertaken
by
the
OCC,
and
reviewed
by
the
EPA,
show
that
the
Hunton
Formation
is
in
fact
under-­‐pressured.
Initial
results
from
a
separate
test
in
the
area
of
interest
indicate
that
the
Arbuckle
Formation
is
also
under-­‐pressured.
Conclusions:
• The
interpretation
that
best
fits
current
data
is
that
the
Prague
Earthquake
Sequence
was
the
result
of
natural
causes.
• The
Prague
Earthquake
Sequence,
as
well
as
other
current
and
historically
active
seismic
areas
in
Oklahoma,
would
benefit
from
further
study,
including
improved
earthquake
monitoring
and
acquisition
of
formation
pressure
data.
• Further
studies
of
seismic
activity
in
Oklahoma
will
result
in
a
better
understanding
of
seismicity
statewide.